La Conti and Maget in the Journal of the Electrochemical Society: Electrochemical Technology 118--pages 506-510 (1971) disclose an electrochemical sensor for detecting H.sub.2, CO and hydrocarbons in inert or oxygen-containing atmospheres. The sensor is constituted by a gas diffusion electrode, using platinum on boron carbide, a sulfuric acid electrolyte and counter electrode constituted by lead dioxide (PbO.sub.2) for use as a CO detector. This electrode system was biased at a content 0.71 V from an external source. The electrochemical sensor had a sensitivity of 500 ppm of CO and the authors concluded that a reasonable goal is to reduce the detection limit to 100 ppm. However, since the Environmental Protection Agency (EPA) has established 50 ppm as a tolerance level, the inability of the La Conti cell to detect below 100 ppm would exclude that device from further practical consideration.
La Conti and Maget erroneously focus their approach on the operation of the electrode as an oxygen electrode; thus, they first establish the potential at which the oxygen reaction current is zero and then they test the sensitivity of the electrode towards CO oxidation at such pre-established potential, dictated by the electrochemistry of oxygen on platinum; consequently, they found a lower limit of sensitivity of about 500 ppm CO in air.
According to the present invention, a limit of detection approximately 2 ppm can be achieved by selecting and appropriately biasing a gas diffusion electrode according to the principles described below. This is comprable to commercially available three electrode systems such as shown in Oswin et al. U.S. Pat. No. 3,776,832.
Carbon monoxide (CO) can be oxidized electrochemically at an anode according to the reaction: EQU CO.sub.(gas) +H.sub.2 O.fwdarw.CO.sub.2(gas) +2H.sup.+ +2e.sup.-( 1)
This reaction occurs in an electrochemical cell, coupled to an electrochemical reduction taking place at the cathode: EQU reducible species+2e.sup.- .fwdarw.reduced products (2)
The nature of the cathodic materials involved in (2) will be discussed below.
It is known that the rate of the overall cell reaction (reactions 1 and 2 coupled) is measured by the current (I) circulating in the external circuit which connects the anode to the cathode. The value of the current is dependent upon several parameters, of which the most relevant are: (i) the potential of the anode (V.sub.a), (ii) the potential of the cathode (V.sub.c), (ii) the concentration (or partial pressure) of the CO (P.sub.co), (iv) the concentration of the reducible species (C.sub.r), (v) the temperature (T), and (vi) the concentration of other species in solution.
The conditions that allow the electrochemical cell to function as a detector of CO are such that the effect of factors (i), (ii), (iv), (v), and (vi) is rendered negligible, thereby leading to a one-to-one relationship between current (I) and CO concentration (P.sub.co).
The effect of anode potential (V.sub.a) on the rate of CO oxidation (or current, I) can be understood with reference to FIG. 1 of the drawings.
In region (a) no reaction takes place (or the rate of the reaction is exceedingly small); in region (b) the rate increases as the anode potential is increased, i.e., the reaction is activated by the electrode polarization and increasing amounts of the available CO are consumed; in region (c) all the available CO is consumed, and an increase in V.sub.a within this region does not lead to an increase in I (limiting current conditions); in region (d) a new reaction takes place in addition to CO oxidation, namely, water discharge: EQU H.sub.2 O.fwdarw.1/2O.sub.2 (gas)+2H.sup.+ +2e.sup.- ( 3)
Region (e) is observed if a reducible species is present in the gas mixture; thus, in air a reduction takes place at the electrode, namely, oxygen reduction: EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O (4)
Clearly, the only region appropriate for the quantitative detection of CO is region (c), where no parallel reactions (water discharge or oxygen reduction) interfere and in which the current is independent of the potential for CO oxidation. The value of V.sub.a in region (c) is from about 1.1 volts to about 1.3 volts versus a reversible hydrogen electrode in the same solution, but the exact values depend on the detailed structure of the anode.
The cell is biased by means of a constant potential .DELTA.V applied between the cathode and anode; thus, the anode potential, V.sub.a, is given by EQU V.sub.a =V.sub.c +.DELTA.V (5)
For a non-polarizable electrode, the slope of the I vs. V relation in region (a) is such that for all values of I of interest, there is no appreciable departure of V.sub.c from the value corresponding to zero current, namely, the reversible potential, E.sub.c, of the electrode.
On the other hand, consideration must be given to the fact that the value of E.sub.c is dependent upon the concentration of reducible and other species in solution (as governed by the well-known Nernst equation). Therefore, the value of V.sub.a cannot be kept strictly constant, V.sub.c being somewhat dependent on current and on concentration of one or several species in solution. For a PbO.sub.2 /PbSO.sub.4 electrode the value of E.sub.c is a function of the concentration (strictly, the activity) of sulfuric acid in the solution.
For these reasons, the biasing to the current limited portion (region (c)) to operate the sensing electrode is essential for the success of a detector: were the electrode to be used in region (b), the small variations in V.sub.c resulting from a slightly polarizable character of the cathode, or even local variations in concentration in the vicinity of the cathode during operation, would result in large variations in current and hence inaccurate detection.
In addition, the sensitivity of the sensing electrode is greatest in region (c), while in region (b) the sensitivity may decrease ten-fold or more for each 120 mV decrease in the value of V.sub.a. Thus, the operation of the sensing electrode in region (c) is a basic element of the detector described herein.
Returning now to the La Conti et al reference, their electrode was not operated in the limiting current region; indeed, from FIG. 7 in that paper it can be seen that the current-voltage curve for CO oxidation was not determined at the potential at which their sensing electrode was operated, the choice of that potential being made on quite different theoretical principles and by a different experimental method; briefly, their method consisted in determining in the absence of CO, the potential V.sub.a,o at which the current through the sensing electrode was exactly zero, thereafter operating the electrode in the presence of CO at that same potential. The rationale of that method is connected with the side reactions referred to in our FIG. 1 as causing regions (d) and (e) of the curve.
It is immediately apparent from FIG. 1 that it is unnecessary to choose V.sub.a exactly equal to V.sub.a,o to achieve a practical detection system. In fact, because of the nature of the side reactions involved (oxygen reduction in region (e) and oxygen evolution in region (d) there is a region of potentials (approximately from 0.9 volts to 1.4 volts vs. a reversible hydrogen electrode in the same solution) in which the rate of oxygen reaction or oxygen evolution is negligibly low. The residual current observed in the absence of CO between 0.9 and 1.4 volts is mainly due to the formation or reduction of an oxide layer on the electrode and the value of that current is much smaller than the CO oxidation current, provided the latter is in the limiting current range.
In short, the much higher sensitivity of the detector described herein compared to that of La Conti et al, is due to the basic difference in the principles of operation of the sensing electrode and operating potential. In this connection, while Teflon bonded noble metal catalyst electrodes on a Teflon membrane support are known in the art, the present invention adopts same in a unique way to achieve a high degree of sensitivity.